body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: #000;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

@keyframes rotate {
  100% {
    transform: rotate(1turn);
  }
}

.conic {
  position: relative;
  z-index: 0;
  width: 400px;
  height: 300px;
  margin: 20px;
  border-radius: 10px;
  overflow: hidden;
  padding: 2rem;

  &::before {
    content: "";
    position: absolute;
    z-index: -2;
    left: -50%;
    top: -50%;
    width: 200%;
    height: 200%;
    background-color: #1a232a;
    background-repeat: no-repeat;
    // background-position: 0 0; // 好像不起作用
    background-image: conic-gradient(transparent, rgba(168, 239, 255, 1), transparent 30%);
    animation: rotate 4s linear infinite;
  }

  &::after {
    content: "";
    position: absolute;
    z-index: -1;
    left: 6px;
    top: 6px;
    width: calc(100% - 12px);
    height: calc(100% - 12px);
    background: #000;
    border-radius: 5px;
  }
}

.conic-demo::after {
  animation: opacityChange 5s infinite linear;
}

@keyframes opacityChange {
  50% {
    opacity: 0.5;
  }
  100% {
    opacity: 1;
  }
}
